Wrote this to clean up my Downloads folder. It moves top-level files into subfolders named after their extension. photo.PNG goes into png/, files with no extension go into no_extension/. --dry-run if you want to see what it would do first.
A few details: it doesn't recurse into existing subfolders, extensions are lowercased so you don't get png/ and PNG/ coexisting, and it handles name collisions by appending _1, _2, etc. rather than overwriting.
~150 lines of Rust, no dependencies outside of tempfile in dev.
cargo install --git https://github.com/timfinnigan/groupr groupr ~/Downloads --dry-run